1. Field of the Invention
This invention relates to a micromanipulation method for handling micro particles. More particularly, the invention relates to a method of non-contact micromanipulation using ultrasound, and to a non-contact micromanipulation apparatus implementing said method.
2. Description of the Prior Art
In various fields such as biotechnology, materials development, micromachinery and the like there has been a strong need for a method of manipulating micro particles. This has led to research into and development of such a method. In addition to being commercially advantageous, such a micromanipulation method needs to take into account the special nature of matter at the micro level.
At the micro level, forces such as the forces arising from friction between solids or the viscosity of liquids become more dominant than inertial forces. In addition to this, while microscopic particles of dust and the like can be ignored at the normal, non-microscopic level, they form major obstructions that cannot be ignored when it comes to the manipulation of micro particles. As a result, methods of trapping and moving micro particles do not work well when a manipulator is used that is simply a scaled down version of a mechanical manipulator used to handle objects on a normal scale, that is, on a non-microscopic scale.
To resolve the special problems involved in handling objects at the micro level, a number of non-contact micromanipulation methods have been proposed that use electrostatic force, laser beam radiation pressure or other such forces to effect the desired manipulation. In addition to resolving the above problems, such non-contact methods have the excellent effect of preventing the objects being manipulated from being contaminated by the manipulation. However, each of the methods has its own particular drawbacks.
Among the problems of the method of micromanipulation using electrostatic force are that it has a short functional distance, that electrolysis arises in the electrodes, and that the objects and atmosphere being manipulated are subject to limitations relating to conductivity.
The method of micromanipulation using laser beam radiation pressure is limited to objects that transmit or refract light. Also, the force produced by the laser beam radiation pressure is so small that it can only be used to manipulate extremely small objects. Among further drawbacks are that the method requires costly equipment, and that care must be taken to ensure that the human body is protected from harm.
However, it is well-known that when an object is placed in an ultrasonic wave field formed in a fluid medium, acoustic radiation pressure is produced around the object, with a force acting on the object being dependent on where the object is located. When for example the sound field is a traveling wave, the acoustic radiation pressure causes the object in the medium to be subjected to the force of a pressure acting in the direction of the ultrasonic traveling wave.
In the fluid medium an ultrasonic wave field can be obtained as a one-dimensional standing wave formed by the superposition of two plane waves traveling in opposite directions that have the same frequency and amplitude. It is also well-known that provided the object (hereinafter referred to as a micro particle) is sufficiently smaller than the wavelength of the standing wave, the ultrasonic radiation force produced around the micro particle urges the micro particle toward the nearest of dynamically stable positions located at half wavelength intervals. In the case of round micro particles, the positions of dynamically stable locations are determined by the negative or positive value of prescribed parameters determined by the medium, the micro particles and their density and compressibility. In the case of the positive value, nodes in the sound pressure distribution of the one-dimensional standing wave become dinamically stable locations, whereas antinodes therein become dinamically stable locations in the case of the negative value. For example, when the micro particles have a density greater than the density of the medium and a compressibility that is lower than that of the medium, the nodes will become dynamically stable locations, and conversely, when the micro particles have a density lower than the density of the medium and a compressibility that is higher than that of the medium, the antinodes will become dynamically stable locations.
A number of manipulation methods using the radiation force of an ultrasound traveling or standing wave have been studied. Since in these methods the ultrasonic radiation force is produced by spatial changes in the energy density of the sound waves in the medium, it follows that if it were done in the medium in which the sound waves were propagated, it would be possible to utilize the radiation force. This had the major additional advantage of expanding the range of particles that could be manipulated so long as the particles had a different acoustic impedance from that of the medium and therefore reflected or absorbed the sound waves. Among other advantages are that the cost of the apparatus is relatively low, and that the fact that ultrasonic waves can be blocked by providing a layer of air between the liquid medium and any human body means that it is easy to ensure safety.
In U.S. patent application Ser. No. 726,300 the present inventors disclose manipulation of micro particles using ultrasound. Standing ultrasonic waves were generated in water by using a concave transducer to radiate ultrasonic waves and a reflector to reflect the ultrasonic waves. Using the radiation force of the standing ultrasonic wave fields, alumina particles suspended in the water were agglomerated and trapped at half wavelength intervals in small regions in the vicinity of nodes along the standing wave sound pressure distribution axis of the transducer forming dynamically stable points, and the particle agglomerations thus trapped at each node were then moved along the center axis by varying the frequency of the ultrasonic waves.
With this micromanipulation method the alumina particles can thus be agglomerated and trapped at half wavelength intervals in the vicinity of nodes along the standing wave sound pressure distribution axis of the transducer, and the trapped particles moved along the center axis by varying the intervals between the nodes in the standing wave sound pressure distribution by varying the frequency of the ultrasonic waves. However, drawbacks are that the standing wave field cannot be moved in a direction perpendicular to the center axis, that is, in a direction parallel to the operating surface of the transducer, and that the trapped particles cannot be moved in a direction parallel to the operating surface of the transducer.
An object of the present invention is to provide a method of non-contact micromanipulation using ultrasound that enables micro particles to be trapped at prescribed small regions and enables the trapped micro particles to be moved in a direction that is substantially parallel to the operating surface of the ultrasound transducer, and a non-contact micromanipulation apparatus implementing said method.